The research of the Viral Genetics Section continues to be directed towards studies of type C virus encoded transforming protein. For this purpose, we have used a number of model systems with particular emphasis on Abelson murine leukemia virus (AbLV), and several independently-derived isolates of feline sarcoma virus (FeSV). We have shown such viruses to encode polyproteins of 100,000-170,000 molecular weight containing both structural (p15, p12 and variables extents of p30) and acquired sequence encoded nonstructural components. Characterization of the nonstructural components of polyproteins encoded by AbLV and Gardner FeSV have indicated multiple sites of phosphorylation and tyrosine specific protein kinase-associated activity. The involvement of this enzymatic activity of transformation has been established through the isolation and characterization of transformation-defective mutants. The demonstration of tryptic peptides common to the nonstructural components of polyproteins encoded by independent isolates of FeSV raises the possibility that the frequency of cellular sequences with transforming potential may be relatively limited.